U.S. patent number 6,443,227 [Application Number 09/721,455] was granted by the patent office on 2002-09-03 for azimuth control of hydraulic vertical fractures in unconsolidated and weakly cemented soils and sediments.
This patent grant is currently assigned to Golder Sierra LLC. Invention is credited to Grant Hocking, Samuel L. Wells.
United States Patent |
6,443,227 |
Hocking , et al. |
September 3, 2002 |
Azimuth control of hydraulic vertical fractures in unconsolidated
and weakly cemented soils and sediments
Abstract
A method and apparatus for initiating an azimuth controlled
vertical hydraulic fracture in unconsolidated and weakly cemented
soils and sediments using active resistivity to monitor and control
the fracture initiation and propagation. Separate or overlapping
treatment walls and containment barriers can be created by
controlling and monitoring the propagation of fractures in the
subsurface. A fracture fluid is injected into a well bore to
initiate and propagate a vertical azimuth controlled fracture. The
fracture fluid is energized to conduct electrical current while the
fracture propagates through the ground. A series of electrical
resistivity monitors measure the electrical conductivity of the
fracture fluid in real time against the background conductivity of
the formation. Using a series of incremental influence functions,
the azimuth of the vertical fracture can be controlled by
regulating the injection of fracture fluid based upon the induced
voltage of the earth and the conductivity of the fluid in the
fracture.
Inventors: |
Hocking; Grant (Alpharetta,
GA), Wells; Samuel L. (Lawrenceville, GA) |
Assignee: |
Golder Sierra LLC (Atlanta,
GA)
|
Family
ID: |
22714993 |
Appl.
No.: |
09/721,455 |
Filed: |
November 22, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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193784 |
Nov 17, 1998 |
6216783 |
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Current U.S.
Class: |
166/250.1;
166/298; 405/129.4; 166/308.1 |
Current CPC
Class: |
E21B
43/26 (20130101); E21B 43/267 (20130101); E21B
47/125 (20200501); C09K 8/80 (20130101) |
Current International
Class: |
C09K
8/60 (20060101); E21B 43/267 (20060101); E21B
43/26 (20060101); E21B 43/25 (20060101); C09K
8/80 (20060101); E21B 043/26 (); E21B
043/267 () |
Field of
Search: |
;166/280-283,298,308,376,250.1,305.1
;405/128.15,128.45,128.5,128.7,128.75,129.1,129.35,129.4,129.45,258.1,263,266 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1 934 170 |
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Jan 1971 |
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DE |
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40 22 897 |
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Jan 1992 |
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DE |
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Primary Examiner: Bagnell; David
Assistant Examiner: Singh; Sunil
Attorney, Agent or Firm: Smith, Gambrell & Russell, LLP
Lischer; Dale Hanson; Eric J.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority benefits under 35 U.S.C. .sctn.
120 based on and is a continuation of U.S. patent application Ser.
No. 09/193,784, filed Nov. 17, 1998, now U.S. Pat. No. 6,216,783,
entitled "AZIMUTH CONTROL OF VERTICAL HYDRAULIC FRACTURES IN
UNCONSOLIDATED AND WEAKLY CEMENTED SOILS AND SEDIMENTS." This
parent application is entirely incorporated herein by reference.
Claims
What is claimed is:
1. A method for constructing subsurface structures oriented at a
predetermined azimuth in a formation of unconsolidated and weakly
cemented soil and sediments, comprising: drilling a bore hole with
an axis in the formation of unconsolidated and weakly cemented soil
and sediments to a predetermined depth; installing an injection
casing with an outer surface in the bore hole at the predetermined
depth, wherein an annular space exists between the outer surface of
the casing and the bore hole; filling the annular space with a
grout that bonds to the outer surface of the casing; weakening the
injection casing along a vertical weakening line lying within a
fracture plane, which fracture plane extends from the axis of the
bore hole radially along the predetermined azimuth; injecting a
fracture fluid into the injection casing with a sufficient
fracturing pressure to dilate the injection casing, the grout, and
the formation and thereby initiate a vertical fracture in the
formation at the weakening line; controlling the fracture fluid
based on a sequential incremental solution to determine incremental
fracture geometry changes as the fracture is propagating; and
forming a wall-like subsurface barrier layer with the propagating
fracture fluid, wherein such barrier layer remains in place for the
ongoing treatment or containment of a predetermined material
permeating to the barrier layer through the formation of
unconsolidated and weakly cemented soil and sediments.
2. The method of claim 1, wherein installing the injection casing
comprises the following step: installing an initiation section of
the injection casing at a predetermined depth in the bore hole,
wherein the initiation section is weakened along the weakening line
so that the initiation section separates under the fracturing
pressure, whereby the fracture fluid dilates the grout and the
formation to initiate the fracture in the formation at the
weakening line.
3. The method of claim 1, wherein the fracture fluid does not leak
off into the formation from the fracture.
4. The method of claim 1, wherein the fracture fluid comprises a
proppant, and the fracture fluid is able to carry the proppant of
the fracture fluid at low flow velocities.
5. The method of claim 1, wherein the fracture fluid is clean
breaking with minimal residue.
6. The method of claim 1, wherein the fracture fluid has a low
friction coefficient.
7. The method of claim 1, wherein the fracture fluid comprises a
water based guar gum gel.
8. The method of claim 1, wherein the fracture fluid comprises a
bentonite cement slurry.
9. The method of claim 1, wherein the fracture fluid comprises a
proppant to create a containment wall contained within the vertical
fracture.
10. The method of claim 1, wherein the fracture fluid comprises a
proppant to create a treatment barrier contained within the
vertical fracture.
11. The method of claim 1, wherein the method further includes
forming a plurality of bore holes with injection casings therein
and creating a plurality of vertical fractures that
interconnect.
12. The method of claim 11, wherein the fracture fluid comprises a
proppant for creating a continuous containment wall contained
within the interconnected plurality of vertical fractures.
13. The method of claim 11, wherein the fracture fluid comprises a
proppant for creating a continuous treatment barrier contained
within the interconnected plurality of vertical fractures.
14. The method of claim 1, wherein the injection of the fracture
fluid is controlled based upon a sequential incremental solution to
determine incremental fracture geometry changes as the fracture is
propagating and based on the measured conductivity of the fracture
fluid.
Description
TECHNICAL FIELD
The present invention generally relates to constructing subsurface
structures by injecting a fracture fluid to fracture underground
formations, and more particularly to a method and apparatus for
creating a vertical fracture oriented at a predetermined azimuth in
unconsolidated and weakly cemented soils and sediments resulting in
subsurface containment walls and or treatment barriers.
BACKGROUND OF THE INVENTION
Subsurface structures are installed to perform various
environmental, geotechnical, and petroleum recovery functions. In
the case of environmental and geotechnical applications,
containment walls and treatment barriers are typically installed to
extend from the ground surface to a subsurface zone. In these
applications, the containment walls may include flow containment
walls which contain the flow of underground liquids, and treatment
barriers which are permeable zones filled with reactive material.
In many cases, the construction process must penetrate many feet
below the ground surface before reaching a subsurface zone that
requires a structure such as a containment wall or a treatment
barrier. Examples of construction techniques performed in the prior
art are sheet piling walls, slurry walls, braced excavations, and
continuous trenches.
Current construction techniques required to install the above
containment walls and treatment barriers share many common
problems, such as the necessity to reroute underground utilities,
potential structural damage to existing buildings and structures,
potentially large staging areas for construction equipment,
specialized and expensive equipment. In many cases, the removal and
proper disposal of contaminated soils and liquids recovered from
the excavation is required. Most of the above examples, for either
economical or technical reasons, have a maximum wall or barrier
depth that may not allow a project to be completed or even
begun.
Turning now to the prior art, hydraulic fracturing of subsurface
earth formations to stimulate production of hydrocarbon fluids from
subterranean formations has been carried out in many parts of the
world for over fifty years. The earth is hydraulically fractured
either through perforations in a cased well bore or in an isolated
section of an open bore hole. The horizontal and vertical
orientation of the hydraulic fracture is controlled by the
compressive stress regime in the earth and the fabric of the
formation. It is well known in the art of rock mechanics that a
fracture will occur in a plane perpendicular to the direction of
the minimum stress, see U.S. Pat. No. 4,271,696 to Wood. At
significant depth, one of the horizontal stresses is generally at a
minimum, resulting in a vertical fracture formed by the hydraulic
fracturing process. It is also well known in the art that the
azimuth of the vertical fracture is controlled by the orientation
of the minimum horizontal stress.
At shallow depths, the horizontal stresses could be less or greater
than the vertical overburden stress. If the horizontal stresses are
less than the vertical overburden stress, then vertical fractures
will be produced; whereas if the horizontal stresses are greater
than the vertical overburden stress, then a horizontal fracture
will be formed by the hydraulic fracturing process.
Techniques to induce a preferred horizontal orientation of the
fracture from a well bore are well known. These techniques include
slotting, by either a gaseous or fluid jet under pressure, to form
a horizontal notch in an open bore hole. Such techniques are
commonly used in the petroleum and environmental industry. The
slotting technique performs satisfactorily in producing a
horizontal fracture, provided that the horizontal stresses are
greater than the vertical overburden stress, or the earth formation
has sufficient horizontal layering or fabric to ensure that the
fracture continues propagating in the horizontal plane.
Perforations in a horizontal plane to induce a horizontal fracture
from a cased well bore have been disclosed, but such perforations
do not preferentially induce horizontal fractures in formations of
low horizontal stress. See U.S. Pat. No. 5,002,431 to Heymans.
Various means for creating vertical slots in a cased well bore have
been disclosed. The prior art recognizes that a chain saw can be
used for slotting the casing. See U.S. Pat. No. 1,789,993 to
Switzer; U.S. Pat. No. 2,178,554 to Bowie, et al., U.S. Pat. No.
3,225,828 to Wisenbaker; and U.S. Pat. No. 4,119,151 to Smith.
Installing pre-slotted or weakened casing has also been disclosed
in the prior art as an alternative to perforating the casing. See
U.S. Pat. No. 5,103,911 to Heijnen. These methods in the prior art
were not concerned with the azimuth orientation of two opposing
slots for the preferential initiating of a vertical hydraulic
fracture at a predetermined azimuth orientation. It has been
generally accepted in the art that the fracture azimuth orientation
cannot be controlled by such means. These methods were an
alternative to perforating the casing to achieve better connection
between the well bore and the surrounding formation.
In the art of hydraulic fracturing subsurface earth formations from
subterranean wells at depth, it is well known that the earth's
compressive stresses at the region of fluid injection into the
formation will typically result in the creation of a vertical two
"winged" structure. This "winged" structure generally extends
laterally from the well bore in opposite directions and in a plane
generally normal to the minimum in situ horizontal compressive
stress. This type of fracture is well known in the petroleum
industry as that which occurs when a pressurized fracture fluid,
usually a mixture of water and a gelling agent together with
certain proppant material, is injected into the formation from a
well bore which is either cased or uncased. Such fractures extend
radially as well as vertically until the fracture encounters a zone
or layer of earth material which is at a higher compressive stress
or is significantly strong to inhibit further fracture propagation
without increased injection pressure.
It is also well known in the prior art that the azimuth of the
vertical hydraulic fracture is controlled by the stress regime with
the azimuth of the vertical hydraulic fracture being perpendicular
to the minimum horizontal stress direction. Attempts to initiate
and propagate a vertical hydraulic fracture at a preferred azimuth
orientation have not been successful, and it is widely believed
that the azimuth of a vertical hydraulic fracture can only be
varied by changes in the earth's stress regime. Such alteration of
the earth's local stress regime has been observed in petroleum
reservoirs subject to significant injection pressure and during the
withdrawal of fluids resulting in local azimuth changes of vertical
hydraulic fractures.
The determination of the hydraulic fracture geometry, such as its
horizontal or vertical orientation, azimuth and length of the
vertical fracture, and the extent and depth of a horizontal
fracture, can be made from the measurement of earth tilts by
conventional surface or bore hole mounted biaxial tiltmeters, see
U.S. Pat. No. 4,271,696 to Wood. Highly sensitive electronic
tiltmeters, capable of measuring tilts less than 10.sup.-7 radians,
measure the earth's deformation due to the opening and propagation
of a hydraulic fracture. From monitoring these tilts in real time
along with the flow of injected fluid, the hydraulic fracture
geometry can be determined. See U.S. Pat. Nos. 4,271,696 and
4,353,244 to Wood. Influence functions that relate the opening of a
fracture to ground deformation can be utilized to calculate the
fracture geometry. As suggested by U.S. Pat. No. 5,002,431 to
Heymans, the fracture geometry can be determined and controlled
from the measurement of tilts and real time computer control.
Heymans does not detail how the fracture geometry may be
determined, nor does Heymans disclose how the interaction of ground
tilts from multiple fractures can be resolved to determine fracture
geometry.
The method of determining the hydraulic fracture geometry,
disclosed by U.S. Pat. No. 4,353,244 to Wood, has a number of
deficiencies. If (a) the fracture is non-planar, (b) if the
fracture is not of the full initiated height, or (c) if multiple
fractures are initiated in close proximity of each other, then
fracture geometry determination is not assured.
Accordingly, there is a need for a method and apparatus for
controlling the azimuth orientation of a vertical hydraulic
fracture in formations of unconsolidated or weakly cemented
sediments and soils.
There is a further need for a method and apparatus for monitoring
and calculating in real time the propagation of the azimuth of
vertical hydraulic fractures.
And, there is a further need for a method and apparatus for the
creation and control of coalesced, overlapping, and interconnecting
fractures to form a treatment barrier or containment wall formed
from a fracture fluid.
SUMMARY OF THE INVENTION
The present invention is a method and apparatus for dilating the
earth by various means from a bore hole or driven lance to initiate
and to control the azimuth orientation of a vertical hydraulic
fracture in formations of unconsolidated or weakly cemented
sediments. The fracture is initiated by means of preferentially
dilating the earth orthogonal to the desired azimuth direction.
This dilation of the earth can be generated by a variety of means:
a driven spade to dilate the ground orthogonal to the required
azimuth direction, packers that inflate and preferentially dilate
the ground orthogonal to the required azimuth direction,
pressurization of a pre-weakened casing with the weaknesses aligned
in the required azimuth orientation, pressurization of a casing
with opposing slots cut along the required azimuth direction, or
pressurization of a two "winged" artificial vertical fracture
generated by cutting or slotting the casing, grout, and/or
formation at the required azimuth orientation.
In addition to initiation of vertical fractures at a predetermined
azimuth, the present invention further provides a method and
apparatus for monitoring and controlling the propagation of the
fractures along the predetermined azimuth. The present invention
relies upon the determination of influence functions that relate
the earth's deformation to the opening of an elementary
fracture.
Using generally accepted principles of elasticity theory, influence
functions such as Green's functions can be applied to the problem.
See "Hydrodynamics" by H. Lamb, 4.sup.th edition, Cambridge (1916)
and "Treatise on Mathematical Theories of Elasticity" by A. E. H.
Love, 4.sup.th edition, Dover Publications (1944). If a series of
Green's functions can be solved relating incremental measurements
of the earth's induced voltage or deformation with the geometry
change in an initiated fracture, then a sequential incremental
solution can be determined. With a series of sequential incremental
solutions, an inverse model can be created for a particular type of
soil and sediment formation or for a specific formation. The
geometry of the fracture can then be calculated during the
injection process by a series of sequential incremental solution
calculations involving the inverse model. Utilizing either measured
ground tilts or measured induced voltages from electrifying the
fracturing fluid, the user monitors the injected flow of the
injected fluid to determine and control the in situ geometry of the
fracture during the injection process.
Thus, by relating measured incremental ground tilts or measured
induced voltages to yield incremental change of the fracture
geometry constrained by the incremental volume of injected fluid
into each fracture, the incremental change in geometry of each
fracture is found by minimizing the differences in the computed and
measured incremental tilts or induced voltages. The essential
difference between the present invention and the prior art is the
utilization of a sequential incremental solution to determine
incremental fracture geometry changes as the fracture is
propagating. Such a method yields the fracture geometry at a
particular time during the injection process. Without a sequential
incremental solution method to determine the in situ fracture
geometry, the system of equations is poorly defined, and a large
range of differing geometry of fractures can fit or yield the same
tilt field.
The present method and apparatus determine the fracture geometry by
active resistivity after initiating a hydraulic fracture in
moderate to highly resistive ground conditions. The fracture fluid
is electrically conductive and is electrically energized by an
alternating electrical source, typically a 100 Hz low voltage
source. Real time instrumentation monitors resistivity receivers
for surface and sub-surface induced voltages of the 100 Hz signal
due to the energized fracture fluid in the propagating fracture.
Surface and sub-surface induced voltages are recorded. By utilizing
potential influence functions of the induced voltage in the earth
due to an elementary electrified fracture, and from an incremental
inverse model constrained by the incremental volume of injected
fluid, the fracture geometry is determined in real time during the
injection process.
The active resistivity method of monitoring fracture geometry
requires that the fracture fluid be at least twenty times more
conductive than the surrounding ground, to ensure a sharp signal
and high contrast between the fracture and the surrounding medium.
In this case, the energized fracture can be approximated as an
electrified sheet, and potential influence functions of the earth's
induced voltages from an elementary electrified fracture can be
formulated. The fracture fluid can be made conductive by the
addition of soluble salts or by the selection of a suitable
fracture fluid that has a high conductivity. Fracture fluids
suitable for this method and apparatus can comprise, but are not
limited to, a water based guar gum gel for a permeable treatment
barrier and a bentonite cement slurry for an impermeable
containment barrier.
From real time monitoring of the earth's induced voltages due to
the propagating electrified fracture and from the flow of injected
fluid into the fracture, the geometry of the fracture can be
determined. Influence functions relate the earth's induced voltage
to the propagation of an elementary electrified fracture. By
solving the inverse problem of measured incremental induced
voltages to yield fracture incremental geometry change constrained
by the incremental volume of injected fluid into each fracture, the
incremental change in geometry of each fracture can be found by
minimizing the differences in the computed and measured incremental
induced voltages.
If the initiated hydraulic fracture is relatively deep, down hole
resistivity receivers are used to obtain a high precision image of
the energized fracture. Active resistivity monitoring has the added
benefit of determining when individual fractures coalesce and thus
become electrically connected. That is, by energizing the fracture
fluid in each injected well bore individually and in unison, the
electrical coalescence of multiple fractures from different well
bores can be clearly recorded and observed. The imaging and
observation of the down hole resistivity data focuses on
quantifying the continuity of the fractures and assessing the
fracture continuity to determine if any holes or gaps are present.
Such monitoring allows construction procedures to be modified to
ensure the hydraulic fractures are installed as planned and allows
contingency measures to be implemented immediately, e.g. an
additional fracture to patch any hole or additional injection
volumes to ensure coalescence or sufficient overlap.
The present invention also pertains to a method for constructing
subsurface structures including containment walls or treatment
barriers by injecting a liquid slurry into an azimuth controlled
hydraulic fracture or fractures to form either a continuous or
overlapping system of fractures. By initiating and propagating
azimuth orientated vertical hydraulic fractures from a series of
bore holes aligned in the required azimuth direction, coalesced and
overlapping fractures are created to form a containment wall or
treatment barrier composed of the fracture fluid. The process of
monitoring and calculating the fracture in situ geometry during the
injection process, enables determination of when to cease injection
or whether to continue injection to achieve the required shape,
extent, coalescence, or degree of overlap of the azimuth orientated
vertical hydraulic fractures.
The fracture fluid used to form the containment walls and treatment
barriers in the vertical fractures has two purposes. First the
fracture fluid must be formulated in order to initiate and
propagate the fracture within the underground formation. In that
regard the fracture fluid has certain attributes. The fracture
fluid fracture fluid should not leak off into the formation, the
fracture fluid should be clean breaking with minimal residue, and
the fracture fluid should have a low friction coefficient.
Second, once injected into the fracture, the fracture fluid forms
the containment wall or the treatment barrier. In that regard, the
fracture fluid comprises a proppant which produces the integrity
for a containment wall or the active component for a treatment
barrier. Such proppants for containment walls may include, for
example, perlite in a bentonite cement slurry. Such proppants for
treatment barriers may include, for example, iron filings. The
proppants are selected and formulated to accomplish the purpose
intended for the containment wall or the treatment barrier.
The present invention is applicable only to formations of
unconsolidated or weakly cemented sediments and soils with low
cohesive strength compared to the vertical overburden stress
prevailing at the depth of the hydraulic fracture. Low cohesive
strength is defined herein as the greater of 200 pounds per square
inch (psi) or 25% of the total vertical overburden stress. The
method is not applicable to consolidated rock formations, in which
the fracture azimuth is controlled by the rock formation stress
regime.
Although the present invention contemplates the formation of
fractures which generally extend laterally away from a vertical or
near vertical well penetrating an earth formation and in a
generally vertical plane in opposite directions from the well, i.e.
a vertical two winged fracture, those skilled in the art will
recognize that the invention may be carried out in earth formations
wherein the fractures and the well bores can extend in directions
other than vertical.
Therefore, the present invention provides a method and apparatus
for controlling the azimuth of a vertical hydraulic fracture in
formations of unconsolidated or weakly cemented sediments and
soils.
The present invention also provides a method and apparatus for the
creation and control of coalesced, overlapping, and interconnecting
fractures to form a containment barrier or treatment wall.
Further, the present invention provides a method and apparatus for
monitoring and calculating in real time the propagation of a
vertical hydraulic fracture.
Other objects, features and advantages of the present invention
will become apparent upon reviewing the following description of
the preferred embodiments of the invention, when taken in
conjunction with the drawings and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of two injection well casings depicting the
design centerline of the fracture wall before the azimuth
controlled vertical hydraulic fractures are initiated.
FIG. 2 is a cross-sectional side view of the installed injection
well casings as shown in FIG. 1.
FIG. 3 is an enlarged plan view of an individual pre-fracture
injection well casing as shown in FIGS. 1 and 2.
FIG. 4 is an enlarged plan view of an individual pre-fracture
injection well casing with a slot cut through the injection casing
wall and grout, where the slot is aligned with the design
centerline of the fracture wall.
FIG. 5 is a cross-sectional side view of an individual pre-fracture
injection well casing with the length of a vertical slot shown.
FIG. 6 is an enlarged plan view of an individual injection well
casing during initial propagation of the azimuth controlled
vertical fracture.
FIG. 7 is a cross-sectional side view of two injection casings with
winged initiation sections of the injection casing construction for
azimuth controlled vertical fracture initiation.
FIG. 8 is a plan view detail of the fabrication and installation of
the winged initiation sections of the injection casing construction
prior to initiation of the azimuth controlled vertical
fracture.
FIG. 9 is a plan view detail of the initiation of an azimuth
controlled vertical fracture from a winged initiation section of
the injection casing construction.
FIG. 10 is a plan view of an individual injection well casing with
locations of the vertical installed resistivity arrays, parallel
and offset to the design centerline of the azimuth controlled
vertical fracture that has not yet been initiated.
FIG. 11 is a cross-sectional side view showing the construction
detail and arrangement of a resistivity sensor array.
FIG. 12 is a cross-sectional side view of a single resistivity
array showing the locations of individual resistivity
receivers.
FIG. 13 is cross-sectional side view of the initial start of an
azimuth controlled vertical fracture superimposed on FIG. 11.
FIG. 14 is a cross-sectional side view of the final design geometry
of an azimuth controlled vertical fracture superimposed on FIG.
11.
FIG. 15 is the visual display of an arrangement of individual
resistivity receivers in a ground formation prior to initiation of
an azimuth controlled vertical fracture in the subsurface.
FIG. 16 is the visual display showing the measured voltages of the
individual resistivity receivers against background reference
voltages illustrating a propagating azimuth controlled vertical
fracture in the subsurface.
DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENT
Several embodiments of the present invention are described below
and illustrated in the accompanying drawings. The present invention
involves a method and apparatus for initiating and propagating an
azimuth controlled vertical fracture in subsurface formations of
unconsolidated and weakly cemented soils and sediments. In
addition, the present invention involves a method and apparatus for
controlling and monitoring the fracture initiation, propagation,
and geometry of a vertical fracture.
Referring to the drawings, in which like numerals indicate like
elements, FIGS. 1, 2, and 3 illustrate the initial setup of the
method and apparatus. Conventional bore holes 5 and 5' with axes 12
and 12' are completed by an auger, wash rotary, or cable tool
methods into the formation 6 of unconsolidated or weakly cemented
soils and sediments to a predetermined depth 9 below the ground
surface 4 on the predetermined fracture line or plane 3 of the
planned azimuth controlled vertical fracture. The fracture plane 3
extends radially from axis 12 to axis 12'. Injection casings 1 and
2 are installed to the predetermined to depth 9 and completed by
placement of a grout 7 which completely fills the annular space
between the outside the injection casings 1 and 2 and the bore
holes 5 and 5'. Injection casings 1 and 2 must be constructed from
a material that can withstand the pressures that the fracture fluid
exerts upon the interior of the injection casings 1 and 2 during
the pressurization of the fracture fluid. The grout 7 can be any
conventional material that preserves the spacing between the
exterior of the injection casings 1 and 2 and the bore holes 5 and
5' throughout the fracturing procedure, preferably a non-shrink or
low shrink cement based grout.
The outer surface of the injection casings 1 and 2 should be
roughened or manufactured such that the grout 7 bonds to the
injection casings 1 and 2 with a minimum strength equal to the down
hole pressure required to initiate an azimuth controlled vertical
fracture. The bond strength of the grout 7 to the outside surface
of the casings 1 and 2 prevents the pressurized fracture fluid from
short circuiting along the casing-to-grout interface up is to the
ground surface 4.
Referring to FIGS. 4 and 5, a casing cutter can be lowered into the
injection casing 2 to a predetermined depth 11, and the injection
casing 2 is cut vertically and parallel to the fracture plane 3 of
the planned azimuth controlled vertical fracture. The lateral depth
of cut shown in FIG. 4 is completely through the wall of injection
casing 2 and through the majority of the grout 7. The length 10 of
the vertical cut 8 into the injection casing 2 and the grout 7 is
dependent upon the required size of the planned azimuth controlled
vertical fracture, the site specific soil 6 type, and the in situ
stress conditions within the soil 6. The lateral depth of the
vertical cut 8 to control the azimuth of the vertical fracture may
require laterally extending the vertical cut 8 into the surrounding
soil 6. When this additional depth of vertical cut 8 is necessary,
the resulting notch into the soil 6 is kept open prior to
initiation of the vertical fracture.
Referring to FIG. 6, once the fracture is initiated, injection of a
fracture fluid 20 through the injection casing 2, vertical cut 8,
grout 7, and into the initiated fracture 21 can be made by any
conventional means to pressurize the fracture fluid 20. The
conventional means can include any pumping arrangement to place the
fracture fluid 20 under the pressure necessary to transport the
fracture fluid 20 into the initiated fracture 21 to assist in
fracture propagation and to create a subsurface containment wall or
treatment barrier. For successful fracture initiation and
propagation to the desired size with maximum spacing of the
injection casings 2, the preferred embodiment of the fracture fluid
20 should have the following characteristics.
The fracture fluid 20 should not excessively leak off or lose its
liquid fraction into the adjacent unconsolidated soils and
sediments. The fracture fluid 20 should be able to carry the solids
fraction (the proppant) of the fracture fluid 20 at low flow
velocities that are encountered at the edges of a maturing azimuth
controlled vertical fracture. The fracture fluid 20 should have the
functional properties for its end use such as longevity, strength,
reactivity, permeability, etc. The fracture fluid 20 should be at
least twenty times more conductive than the unconsolidated or
weakly cemented soils and sediments in order to obtain sufficient
electrical contrast for active resistivity methods for tracking the
in situ geometry of the propagating azimuth controlled vertical
fracture.
Permeable groundwater treatment barriers can be constructed by
orientated vertical hydraulic fractures filled with a fracture
fluid 20 having a treatment proppant. Treatment proppants can be
reactive, absorbent or adsorbent, precipitates or biodegradable,
depending upon the groundwater contaminants of concern. Reactive
proppants can comprise, without limitation, the following: iron
filings for the dechlorination of chlorinated solvents, such as
organic contaminants: trichloroethane (TCE), cis 1,2-dichloroethane
(cDCE), tetrachloroethene (PCE), 1,1-dichloroethene (11DCE), 1,1,1
Trichloroethane (111TCA), chloroform (TCM), carbon tetrachloride
(CT) and vinyl chloride (VC). Absorbent or adsorbent proppants can
comprise, without limitation, the following: activated alumina,
activated carbon and resins for the absorption of metals,
phosphate, nitrate, hydrocarbons, etc. Precipitation proppants
comprise, without limitation, the following: iron filings for metal
precipitation, and lime and slag for phosphate removal.
Biodegradation proppants can comprise, without limitation, the
following: enzyme, microbes, nutrients, growth substrates, etc. to
achieve in situ biodegradation of the particular contaminant.
The fracture fluid 20 should be compatible with the proppant, the
subsurface formation, and the formation fluids. Further, the
fracture fluid 20 should be capable of controlling the viscosity of
the proppant, and for carrying the proppant through the formation
fracture. The fracture fluid 20 should be an efficient fluid, i.e.
low leak off from the fracture into the formation, to be clean
breaking with minimal residue, and to have a low friction
coefficient. The fracture fluid 20 should not excessively leak off
or lose its liquid fraction into the adjacent unconsolidated
formation. For a permeable treatment barrier, the gel composed of
starch should be capable of being degraded leaving minimal residue
and not impart the properties of the fracture proppant. A low
friction coefficient fluid is required to reduce pumping head
losses in piping and down the well bore. When a hydraulic fracture
permeable treatment barrier is desired, typically a gel is used
with the proppant and the fracture fluid. Preferable gels can
comprise, without limitation of the following: a water-based guar
gum gel, hydroxypropylguar (HPG), a natural polymer or a
cellulose-based gel, such as carboxymethylhydroxyethylcellulose
(CMHEC). The gel is chosen for its minimal impact on the proppant
material properties, such as reactivity, absorption, etc., and the
extremely low gel residue in the treatment barrier once the enzyme
has been degraded.
The gel is generally cross-linked to achieve a sufficiently high
viscosity to transport the proppant to the extremes of the
fracture. Cross-linkers are typically metallic ions, such as
borate, antimony, zirconium, etc., disbursed between the polymers
and produce a strong attraction between the metallic ion and the
hydroxyl or carboxy groups. The gel is water soluble in the
uncrossed-linked state and water insoluble in the cross-linked
state. While cross-linked, the gel can be extremely viscous thereby
ensuring that the proppant remains suspended at all times. An
enzyme breaker can be added to controllably degrade the viscous
cross-linked gel into water and sugars. The enzyme typically takes
a number of days to biodegrade the gel, and upon breaking the
cross-link and degradation of the gel, a permeable treatment wall
of the proppant remains in the ground with minimal gel residue. For
certain proppants, pH buffers can be added to the gel to ensure the
gel's in situ pH is within a suitable range for enzyme activity.
Salts comprising, but not limited to, sodium chloride, potassium
chloride, and potassium bromide are added to the gel to achieve a
sufficiently high gel electrical conductivity for mapping of the
fracture geometry by the active resistivity method.
The fracture fluid-gel-proppant mixture is injected into the
formation and carries the proppant to the extremes of the fracture.
Upon propagation of the fracture to the required lateral and
vertical extent, the predetermined fracture thickness may need to
be increased by utilizing the process of tip screen out. This
process involves modifying the proppant loading and/or fracture
fluid 20 properties to achieve a proppant bridge at the fracture
tip. The fracture fluid 20 is further injected after tip screen
out, but rather then extending the fracture laterally or
vertically, the injected fluid widens the fracture.
Impermeable flow containment walls constructed by oriented vertical
hydraulic fracturing are typically composed of, without limitation,
bentonite cement slurries with or without special additives for
improving leak off performance, delaying setting time, and reducing
water-to-cement ratios. The bentonite acts as the prime filter cake
building material in the fracture fluid 20, but can be replaced by
alternative materials, without limitation, such as silica flour and
perlite. Generally, bentonite cement slurry has a sufficiently high
electrical conductivity for the active resistivity mapping
technique.
The density of the fracture fluid 20 can be altered by increasing
or decreasing the proppant loading or modifying the density of the
proppant material. In many cases, the fracture fluid 20 density
will be controlled to ensure the fracture propagates downwards
initially and achieves the required height of the planned
structure. Such downward fracture propagation requires the gel
density to be typically greater than 1.25 gm/cc.
The viscosity of the fracture fluid 20 should be sufficiently high
to ensure the proppant remains suspended during injection into the
subsurface, otherwise dense proppant materials will sink or settle
out and light proppant materials will flow or rise in the fracture
fluid 20. The required viscosity of the fracture fluid 20 depends
on the density contrast of the proppant and the gel and the
proppant's maximum particulate diameter. For medium grain-size iron
filings, that is of grain size similar to a medium sand, a fracture
fluid 20 viscosity needs to be typically greater than 100
centipoise at a shear rate of 1/sec.
Referring to FIGS. 4, 5, and 14, the fracture is initiated by
pumping the fracture fluid 20 with a pumping system 104 through the
injection casing 2, 91 to the previously slotted injection casing
2, 91 and grout 7, 106. As best shown in FIG. 4, when the pressure
of the fracture fluid 20 increases, the fracture fluid 20 will
exert lateral forces 19 on the interior of the injection casing 2
and the interior of the vertical cut 8. The lateral forces 19 will
be perpendicular to the fracture plane 3 of the planned azimuth
controlled vertical fracture. The injection casing 2 and grout 7
are shown to be separating in the direction perpendicular to the
fracture plane 3 of the planned azimuth controlled vertical
fracture.
As best shown in FIG. 6, when the pressure of the fracture fluid 20
is increased to a level which exceeds the lateral earth pressures,
the grout 7 which is bonded to the injection casing 2 will begin to
dilate the adjacent soil 6 forming a parting 21 of the soil 6 along
the fracture plane 3 of the planned azimuth controlled vertical
fracture. The fracture fluid 20 rapidly fills the parting 21 of the
soil 6 in the vertical cut 8. Within the injection casing 2, the
fracture fluid 20 exerts normal forces on the soil 6 perpendicular
to the fracture plane 3 which progressively extends the parting 21
and continues to maintain the required azimuth of the initiated
fracture. The azimuth controlled vertical fracture will be expanded
by continuous pumping of the fracture fluid 20 until the desired
geometry is achieved.
In another embodiment for azimuth controlled vertical fracture
initiation refer to FIGS. 7, 8, and 9. As best shown in FIG. 7,
conventional bore holes 51 can be completed by an auger, wash
rotary, or cable tool methods below the ground surface 57 to the
required depth. Injection casings 50 along with wing initiation
sections 52, 53, 54, 55 are installed at predetermined depths
within the bore holes 51. The wing initiation sections 52, 53, 54,
55 can be constructed from the same material as the injection
casings 50. The wing initiation sections 52, 53, 54, 55 are aligned
parallel and through the fracture plane 83 of the pre-constructed
azimuth controlled vertical fracture. The alignment of the wing
initiation sections 52, 53, 54, 55 to the fracture plane 83 can be
performed by a conventional down hole camera with an attached
magnetic compass or by a down hole gyroscopic instrument before the
grout 56 is placed in the annular space between the bore hole 51
and the injection casings 50 and the wing initiation sections 52,
53, 54, 55. The outer surface of the injection casings 51 and of
the wing initiation sections 52, 53, 54, 55 should be a roughened
or manufactured surface such that the bond of the grout 56 is
greater than the fracture initiation pressure. The position below
ground surface of the winged initiation sections 52, 53, 54, 55
will depend on the required in situ geometry site specific soil
properties and the in situ soil stresses.
The winged initiation sections 52, 53, 54, 55 are preferably
constructed from two symmetrical halves 84, 85 as shown in FIGS. 8
and 9. The configuration of the winged initiation sections 52, 53,
54, 55 is not limited to the shape shown, but the chosen
configuration must permit the initiating fracture to propagate
laterally in at least two opposing directions away from the
fracture plane 83 of the winged initiation sections 52, 53, 54, 55.
In FIG. 8, prior to initiating the fracture, the two symmetrical
halves 84, 85 of the winged initiation sections 52, 53, 54, 55 are
connected together by shear fasteners 81 and the two symmetrical
halves 84, 85 are sealed by gasket 80. The gaskets 80 and the shear
fasteners 81 are designed to keep the grout 56 from leaking into
the interior of the winged initiation sections 52, 53, 54, 55
during the grout 56 placement. Furthermore, the gaskets 80 and
shear fasteners 81 are designed to separate along the fracture
plane 83 of the winged initiation sections 52, 53, 54, 55 during
fracture initiation, as shown in FIG. 9, without physical damage to
the two symmetrical halves 84, 85 of the winged initiation sections
52, 53, 54, 55. Any means of connecting the two symmetrical halves
84, 85 of the winged initiation sections 52, 53, 54, 55 can be
made, including but not limited to clips, glue, or weakened
fasteners, as long as the pressure keeping the two symmetrical
halves 84, 85 together is greater than the pressure of the grout 56
on the exterior of the winged initiation sections 52, 53, 54, 55,
i.e. the grout 56 is prevented from leaking into the interior of
the winged initiation sections 52, 53, 54, 55. When the wall and or
barrier geometry requires that the fractures be initiated and
propagated from discrete soil zones 60 in the same injection casing
50, individual winged initiation sections 52, 53, 54, 55 or
vertical cuts 8 in the injection casing 50 can be isolated with
mechanical or inflatable packers 70, 71 prior to fracture
initiation.
Referring to FIG. 7, two embodiments of the method are shown. In
the first embodiment on the right, when the soil zone 60
surrounding the lower winged initiation section 55 requires that
fracture initiation and propagation begin below upper winged
initiation section 53, a single isolation packer 72 is set in
injection casing 50 just above the winged initiation section 55.
The fracture fluid 20 is pumped from the pumping/system operation
104, see FIG. 14, into the pressure pipe 59 and through the single
isolation packer 72. As the fracture fluid 20 pressure increases
below the single isolation packer 72, the azimuth controlled
vertical fracture is initiated and propagated as previously
described. To initiate an azimuth controlled vertical fracture in
the soil zone 60 around winged initiation section 52, upper and
lower isolation packers 70, 71 are positioned in the injection
casing 50 and set above and below the winged initiation section 52,
as shown by the embodiment on the left. The upper isolation packer
70 is connected to lower isolation packer 71 by a perforated pipe
74. The bottom of the lower isolation packer 71 is plugged to
prevent fracture fluid 20 from flowing through the lower isolation
packer 71. With both isolation packers 70, 71 set, the fracture
fluid 20 is pumped from the pumping/system operation 104, see FIG.
14, into a pressure pipe 59 through the upper isolation packer 70,
and exits into the upper winged initiation section 52 from the
perforations in the perforated pipe 74. As the pressure of the
fracture fluid 20 increases between the set upper and lower
isolation packers 70, 71, the azimuth controlled vertical fracture
is initiated and propagated as previously described.
Referring to FIG. 9, as the pressure of the fracture fluid 20 is
increased to a level which exceeds the lateral earth pressures, the
two symmetrical halves 84, 85 of the winged initiation sections 84,
85 will begin to separate along the fracture plane 83 of the winged
initiation sections 84, 85 during fracture initiation without
physical damage to the two symmetrical halves 84, 85 of the winged
initiation sections 84, 85. The gaskets 80 and shear fasteners 81
fracture along the fracture plane 83 of the winged initiation
sections 84, 85 during fracture initiation, as shown in FIG. 9,
without physical damage to the two symmetrical halves 84, 85 of the
winged initiation sections 84, 85. During separation of the two
symmetrical halves 84, 85 of the winged initiation sections 84, 85,
the grout 56, which is bonded to the injection casing 50, see FIG.
7, and the two symmetrical halves 84, 85 of the winged initiation
sections 84, 85 will begin to dilate the adjacent soil 60 forming a
parting 89 of the soil 60 along the fracture plane 83 of the
planned azimuth controlled vertical fracture. The fracture fluid 20
rapidly fills the parting 89 of the soil 60 by the initiated
fracture. Within the two symmetrical halves 84, 85 of the winged
initiation sections 84, 85, the fracture fluid 20 exerts normal
forces 86 on the soil 60 perpendicular to the fracture plane 83 and
opposite to the soil 60 horizontal stresses 87. Thus, the fracture
fluid 20 progressively extends the parting 89 and continues to
maintain the required azimuth of the initiated fracture. The
azimuth controlled vertical fracture will be expanded by continuous
pumping of the fracture fluid 20 until the desired geometry is
achieved.
After initiation of the azimuth controlled vertical fracture and in
order to determine when the fracture has achieved the desired in
situ geometry, real time active resistivity tracking methods or a
conventional tiltmeter tracking method can be employed. Referring
to FIGS. 10, 11, and 12, a real time active resistivity tracking
method and apparatus is shown. In FIG. 10, an individual injection
casing 91 is shown in a top plan view. In most applications
multiple injection casings 91 would be required to employ a real
time tracking method. Generally, vertical arrays 90 of resistivity
receivers 115, 121, 127, 133, as shown in FIG. 12, are located
parallel to fracture plane 92 of the planned azimuth controlled
vertical fracture, but offset on either one or both sides are the
locations of the vertical arrays 90 of resistivity receivers 115,
121, 127, 133.
Referring to FIG. 11, six resistivity receiver arrays 90 are
illustrated in a sectional view from FIG. 10. Each array 90
comprises a plurality of individual receivers 110-133 vertically
connected as shown in FIG. 12 and spaced at depths parallel to the
bore hole 105. The injection casing 91 along with the winged
initiation section 107 is shown offset and parallel to the
connecting plane 92 of the vertical resistivity arrays 90 for
clarity. As shown in FIG. 12, the insulated conductors 201, 202,
203, 204 for each individual resistivity receiver 115, 121, 127,
133 are connected to a data acquisition system 102 seen in FIG. 11.
The data acquisition system 102 is comprised of a multi-channel
electronic switching system (multiplexer), an analog to digital
converter (A to D), and a storage device that stores the incoming
data. The data acquisition system 102 is coupled to a computer 103
where incoming data is analyzed by an installed software
program.
Referring to FIG. 12, the vertical array 90 of resistivity
receivers 115, 121, 127, 133 is installed in conventional bore
holes 211. Typically, the vertical array 90 of resistivity
receivers 115, 121, 127, 133 is constructed of a plurality of
individual resistivity receivers 115, 121, 127, 133. Each
resistivity receiver 115, 121, 127, 133 is manufactured from an
electrically conductive metal, usually copper, and each resistivity
receiver 115, 121, 127, 133 is attached to an insulated conductor
201, 202, 203, 204 which are routed through a nonconductive pipe or
tubing 95 to the ground surface 93. The individual resistivity
receivers 115, 121, 127, 133 are spaced as required and attached to
the nonconductive pipe or tubing 95. The annular space between the
bore hole 211 and the vertical array 90 of resistivity receivers
115, 121, 127, 133 is backfilled with clean sand 212 to a level
above the uppermost individual resistivity receiver 115. The
remainder of the bore hole 211 is backfilled with a surface seal
210 comprising bentonite clay or a non-shrink or low shrink cement
based grout.
Using influence functions such as Green's functions, the computer
software program can relate incremental measurements of the earth's
induced voltage to the geometry change of an electrically energized
initiated fracture. A series of sequential incremental solutions
from the influence functions can be used to create an inverse
model. The geometry of the fracture can be calculated during the
injection process by solving the series of influence functions
incorporated into the inverse model. Utilizing measured induced
voltages from electrifying the fracture fluid 20, the user can
monitor the injected flow of the fracture fluid 20 to determine and
control the in situ geometry of the fracture during the injection
process.
The computer 103 displays an image of the initiated and propagating
fracture in real time. Control of the pumping system/operation 104
can be accomplished by data input from the computer 103.
To generate the data, a high frequency low voltage electrical
supply 134 is attached to the injection casing 91 which can be
either electrically conductive or connected to an electrically
conductive electrode. The electrically conductive electrode can be
placed inside the injection casing 91 so that the fracture fluid 20
when pumped into the injection casing 91 by the pumping
system/operation 104 will receive and conduct the electrical
voltage. That is, the fracture fluid 20 will become energized by
the electrical voltage. A reference ground electrode 109 is driven
into the ground surface 93 as far as practical from the vertical
arrays 90 and the individual resistivity receivers 110-133. The
reference ground can be, without limitation, a subsurface electrode
or a neighboring initiated fracture. Before the azimuth controlled
vertical fracture is initiated, the injection casing 91 is
electrified. The voltages from the injection casing 91 to the
reference ground electrode 109, and from the resistivity receivers
110-133 to the reference ground electrode 109 are measured, stored,
and displayed by the data acquisition system 102 and computer and
integrated software 103. The measured voltages are then used as
background.
Referring to FIGS. 15 and 16, a visual display showing the
locations of the individual resistivity receivers 200 within a
ground formation is illustrated. In FIG. 16, two bore holes 205,
210 are shown within the arrangement of the individual resistivity
receivers 200. The voltages of the individual resistivity receivers
200 are measured against the background reference voltages, and a
visual display of a propagating azimuth controlled vertical
fracture in the subsurface is shown.
Finally, it will be understood that the preferred embodiment has
been disclosed by way of example, and that other modifications may
occur to those skilled in the art without departing from the scope
and spirit of the appended claims.
* * * * *